The present invention relates to a method for producing a sintered rare earth magnet having a low oxygen content and high density and orientation, and to a high-performance, ring-shaped, R-T-B-based, sintered magnet having a lower oxygen content, a higher density and higher orientation in a polar anisotropic or radially dipolar direction than those of the conventional sintered magnets.
Rxe2x80x94Fexe2x80x94B based, sintered magnets, wherein R is at least one of rare earth elements including Y, are produced by coarsely pulverizing Rxe2x80x94Fexe2x80x94B-based alloys of desired compositions, finely pulverizing them in an inert gas such as nitrogen, etc., molding the resultant fine Rxe2x80x94Fexe2x80x94B-based alloy powder having an average particle size of 1-10 xcexcm in a magnetic field, sintering the resultant green body, and then heat-treating the resultant sintered magnet. To provide the Rxe2x80x94Fexe2x80x94B-based, sintered magnets with improved residual magnetic flux density Br and maximum energy product (BH)max, it is extremely important that the Rxe2x80x94Fexe2x80x94B-based, sintered magnets have reduced oxygen content. In view of this fact, the applicants previously discovered mineral oils and synthetic oils having a function to prevent the oxidation of the fine Rxe2x80x94Fexe2x80x94B-based alloy powder, and proposed a process for producing high-performance, Rxe2x80x94Fexe2x80x94B-based, sintered magnet having a low oxygen content and a high density, which comprises a introducing the fine Rxe2x80x94Fexe2x80x94B-based alloy powder into the above mineral or synthetic oil to form a slury, molding the slurry, degreasing the resultant green body, and then sintering and heat-treating it (see Japanese Patent 2,731,337, etc.).
This production process is advantageous in that the fine power and the green body are covered with the above oil to protect them from the air, thereby substantially suppressing their oxidation. Thus, the Rxe2x80x94Fexe2x80x94B-based, sintered body obtained by degreasing and sintering has as low an oxygen content as corresponding to that of the coarse Rxe2x80x94Fexe2x80x94B-based alloy powder before fine pulverization. Because this production process suffers from little decrease in the percentages of the R elements in the Rxe2x80x94Fexe2x80x94B-based alloy due to oxidation during sintering, a rare earth-rich phase forming a grain boundary phase is well kept in the resultant Rxe2x80x94Fexe2x80x94B-based, sintered body. Therefore, the R content in the green body can be set small, leading to decrease in excess R-rich phase and rare earth oxides and thus increase in a volume ratio of R2Fe14B-type crystal grains (main phase), which constitute a ferromagnetic phase, than he conventional sintered magnets. Therefore, this production process can provide a sintered magnet with remarkably improved Br and (BH)max.
However, there is recently strong demand to miniaturize and reduce in weight magnet-comprising electric appliances such as VCMs, CD pickups, motors for domestic electric appliances, etc., resulting in increasingly higher demand to miniaturize and reduce in weight sintered rare earth magnets used in these electric appliances. Though the process of Japanese patent 2,731,337 was considered satisfactory to this demand, higher Br and (BH)max than those achieved by Japanese patent 2,731,337 are now demanded. The inventors"" research has revealed that there is room for improving the magnetic orientation of the slurry.
Accordingly, an object of the present invention is to provide a method for producing a sintered rare earth magnet having a low oxygen content and high density and orientation.
Another object of the present invention is to provide a polar anisotropic or radially bipolar, R-T-B-based, sintered ring magnet having low oxygen content and high density and orientation.
The method for producing a sintered rare earth magnet comprising the steps of finely pulverizing a coarse rare earth magnet alloy powder to an average particle size of 1-10 xcexcm in a non-oxidizing atmosphere; introducing the resultant fine rare earth magnet alloy powder into a non-oxidizing liquid comprising at least one oil selected from the group consisting of mineral oils, synthetic oils and vegetable oils, and at least one lubricant selected from the group consisting of esters of aliphatic acids and monovalent alcohols, esters of polybasic acids and monovalent alcohols, esters of aliphatic acids and polyvalent alcohols and their derivatives to prepare a slurry; molding the slurry; degreasing the resultant green body; sintering the degreased green body; and then heat-treating the green body.
A weight ratio of the lubricant to the fine rare earth magnet alloy powder is preferably 0.01/99.99 to 0.5199.5.
The sintered rare earth magnet may further comprise 0.01-0.2 weight % of Ga, 0.01-0.3 weight % of Al, and 0.01-0.8 weight % of Nb.
The polar anisotropic ring magnet having is constituted by an Rxe2x80x94Fexe2x80x94Coxe2x80x94Cuxe2x80x94B-based, sintered magnet comprising 28-33 weight % of R, wherein R is at least one rare earth element including Y, 50 atomic % or more of R being occupied by Nd, 0.8-1.5 weight % of B, 0.5-5 weight % of Co, and 0.01-0.3 weight % of Cu, the balance being substantially Fe and inevitable impurities; the amount of oxygen inevitably contained being 0.3 weight % or less based on the total weight of the ring magnet; the ring magnet having a density of 7.56 g/cm3 or more; and a ratio of I (105)/I (006) being 0.5-0.8, wherein I (105) and I (006) are X-ray diffraction peak intensity measured with respect to (105) and (006) planes, respectively, at a middle position on an outer surface between magnetic poles of the ring magnet. With I(105)/I (006)=0.5-0.8, the sintered magnet has higher Br and (B) than those of the conventional sintered magnets.
The radially dipolar ring magnet of the present invention is constituted by an Rxe2x80x94Fexe2x80x94Coxe2x80x94Cuxe2x80x94B-based, sintered magnet comprising 28-33% by weight of R, wherein R is at least one rare earth element including Y, 50 atomic % or more of R being occupied by Nd, 0.8-1.5% by weight of B, 0.5-5 weight % of Co, and 0.01-0.3 weight % of Cu, the balance being substantially Fe and inevitable impurities; the amount of oxygen inevitably contained being 0.3 weight % or less based on the total weight of the ring magnet; the ring magnet having a density of 7.56 g/cm3 or more and an intrinsic coercivity iHc of 1.1 MA/m (14 kOe) or more at room temperature; and the degree of orientation expressed by [(Br//) 1 (Br//+Brixe2x8axa5)]xc3x97100(%) being 85.5% or more, wherein Br// is a residual magnetic flux density in an orientation direction at room temperature, and Brxe2x8axa5 is a residual magnetic flux density in a longitudinal direction perpendicular to the orientation direction.
The thin, sintered arc segment magnet having a thickness of 1-4 mm according to the present invention is constituted by an Rxe2x80x2xe2x80x94Fexe2x80x94B-based, sintered magnet comprising 28-33 weight % of Rxe2x80x2, wherein Rxe2x80x2 is at least one of rare earth elements including Y, 50 atomic % or more of Rxe2x80x2 being occupied by Pr, 0.8-1.5 weight % of B, 5 weight % or less of Co, and 0.3 weight % or less of Cu, the balance being substantially Fe and inevitable impurities; the amount of oxygen inevitably contained being 0.3 weight % or less based on the total weight of the arc segment magnet; the arc segment magnet having a density of 7.50 g/cm3 or more, a coercivity iHc of 1.1 MA/m (14 kOe) or more at room temperature, and orientation (Br/4xcfx80Imax) of 96% or more in an anisotropy direction.
The thin, sintered arc segment magnet preferably has parallel anisotropy and a length of 40-100 mm. Further, a ratio of I (105)/I (006) is preferably 0.5-0.8, wherein I (105) and I (006) are X-ray diffraction peak intensity measured with respect to (105) and (006) planes, respectively, using CuKxcex11-ray (xcex=0.15405 nm). With I (105)/I (006)=0.5-0.8, the thin, sintered arc segment magnet has higher Br and (BH than those of the conventional sintered magnets.
The radially anisotropic, sintered arc segment magnet having an inner diameter of 100 mm or less according to the present invention is constituted by an Rxe2x80x2xe2x80x94Fexe2x80x94B-based, sintered magnet comprising 28-33 weight % of Rxe2x80x2, wherein Rxe2x80x2 is at least one of rare earth elements including Y, 50 atomic % or more of Rxe2x80x2 being occupied by Pr, 0.8-1.5 weight % of B, 5 weight % or less of Co, 0.3 weight % or less of Cu, the balance being substantially Fe and inevitable impurities; the amount of oxygen inevitably contained being 0.3 weight % or less based on the total weight of the arc segment magnet; the arc segment magnet having a density of 7.50 g/cm3 or more and a coercivity iHc of 1.1 MA/m (14 kOe) or more at room temperature; and the degree of orientation expressed by [(Br//)/(Br//+Brxe2x8axa5)]xc3x97100(%) being 85.5% or more, wherein Br// is a residual magnetic flux density in a radial direction at room temperature, and Brxe2x8axa5 is a residual magnetic flux density in a longitudinal direction perpendicular to the radial direction.
This arc segment magnet is preferably as thin as 14 mm and as long as 40-100 mm in an axial direction.
The radially anisotropic, sintered ring magnet having an inner diameter of 100 mm or less according to the present invention is constituted by an Rxe2x80x2xe2x80x94Fexe2x80x94B-based, sintered magnet comprising 28-33 weight % of Rxe2x80x2, wherein Rxe2x80x2 is at least one of rare earth elements including Y, 50 atomic % or more of Rxe2x80x2 being occupied by Pr, 0.8-1.5 weight % of B, 5 weight % or less Co, 0.3 weight % or less of Cu, the balance being substantially Fe and inevitable impurities; the amount of oxygen inevitably contained being 0.3 weight % or less based on the total weight of the ring magnet; the ring magnet having a density of 7.50 g/cm3 or more and a coercivity iHc of 1.1 MA/m (14 kOe) or more at room temperature; and the degree of orientation expressed by [(Br//)/(Br//+Brxe2x8axa5)]xc3x97100(%) being 85.5% or more, wherein Br// is a residual magnetic flux density in a radial direction at room temperature, and Brxe2x8axa5 is a residual magnetic flux density in a longitudinal direction perpendicular to the radial direction.